The tailored integration of nanoparticles as functional building blocks in photovoltaic, optical, electronic, and sensing devices requires macroscopically controlling the morphology of the nanoparticle ensemble. This is of particular interest in the case of anisotropically shaped nanorods, where the collective properties of the plurality of nano-rods strongly depend on rod alignment and internal organization.
A variety of methods to achieve macro-control over spherical nanoparticles have been reported, including close-packed superlattices (Murray et. al. Annu. Rev. Mater. Sci. 2000, 30, 545-610), lithographic methods (Cui et al. Nano Lett. 2004, 4, 1093-1098), and self-assembly techniques (Bockstaller et. al. J. Am. Chem. Soc. 2003, 125, 5276-5277; Chiu et. al. J. Am. Chem. Soc. 2005, 127, 5036-5037; Zhang et. al. Nano Lett. 2005, 5, 357-361).
The self-organization property of block copolymers (BCPs) has been successfully harnessed to direct spherical nanoparticles to specific copolymer domains (Matsen et al., Science 2001, 292, 2469).
Outside the scope of liquid crystalline ordering (Kim et al. J. Am. Chem. Soc. 2001, 123, 4360-4361; Li et al. Adv. Mater. 2003, 15, 408-411; Nano Lett 2002, 2, 557-560. Adv. Mater. 2007, 19, 2073-2078; J. Mater. Chem. 2008, 18, 3050) comparatively little control has been achieved in macroscopic ordering of anisotropic rod and rod-like nano-particles, especially into hierarchical structures, probably due to the complexity of these self-assembly hybrid systems. Factors, such as strong and directional van der Waals attraction between the nano-rods (resulting in the formation of NR aggregates), the tendency to form liquid crystalline phases, and additional orientational entropy of the anisotropic nanoparticles, introduce complexity and challenges to the controls of these systems.
However, current knowledge of the phase behavior of rod-shaped nanoparticles/BCP assemblies is limited mostly to bulk composites and there is little understanding on the behavior of such systems in thin films (Zhang et al. J. Am. Chem. Soc. 2006, 128, 3898; Gon Son et al. ACS Nano 2009 3, 3927).
Deshmukh et al. (Nano Lett. 2007, 7, 3662) assembled gold NRs inside thick films consisting of lamellae of polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA) and found that the nanorods orient parallel to the lamellae direction with limited deviation from the parallel orientation, dictated by the NR length relative to the BCP domain size. Theoretical studies by Balazs et al. (Compos. Interfaces 2003, 10, 343) and Ma et al. (J. Phys. Chem. B 2009, 113, 10117) also concluded that NR orient parallel to the BCP domain axes.
Other methods known in the art for controlling NR orientation, alignment and closed-packed form include: applying an electric field during solvent evaporation (Ryan et al. Nano Lett. 2006, 6, 1479; Gupta et al. Nano Lett. 2006, 6, 2066), stretching polymer films containing NRs (van der Zande et al. J. Phys. Chem. B 1999, 103, 5761) electron beam lithography with a lift-off technique (Ueno et al. Opt. Lett. 2005, 30, 2158), deposition on chemically patterned substrates (Liu et al. Small 2006, 2, 1448) and optical trapping via laser radiation (Pelton et al. Opt. Lett. 2006, 31, 2075). Alignment of a monolayer of silver nanowires on silicon was also achieved using the Langmuir-Blodgett technique (Tao et al. Nano Lett. 2003, 3, 1229).
There is thus a need to provide BCP films comprising organized nano-rods in order to enable the creation of arrays of individually addressable nano-components that make up devices.